In the field of optical communication, a digital coherent communication system which combines a coherent detection scheme and digital signal processing for significantly improving frequency utilization efficiency is drawing attention. Compared to a system built with direct detection, this type of communication system is known to be capable of not only improving receiver sensitivity but also compensating for waveform distortion in a transmitted signal caused by chromatic dispersion and polarization mode dispersion associated with optical fiber transmission, by receiving the transmitted signal as a digital signal. Therefore, introduction of this type of communication system is being considered as a next-generation optical communication technique.
The digital coherent schemes exemplified by Non-Patent Documents 1 and 2 employ a method in which quasi-static chromatic dispersion is compensated for by a digital filter having a fixed number of taps (e.g., the number of taps is 2048 taps for the dispersion of 20000 ps/nm and for a signal at 28 G Baud), and variable polarization-mode dispersion is compensated for by an adaptive filter with a small number of taps, (e.g., about 10 to 12 taps for polarization-mode dispersion of 50 ps) using a blind algorithm.